What is Tensile Testing?
A tensile test,
also known as tension test, is probably the most fundamental type of mechanical
test you can perform on material. Tensile tests are simple, relatively inexpensive,
and fully standardized. By pulling on something, you will very
quickly determine how the material will react to forces
being applied in tension. As the material is being pulled, you will
find its strength along with how much it will elongate.
Why Perform a Tensile Test or Tension Test?
You can learn a lot about a substance from tensile testing.
As you continue to pull on the material until it breaks, you will obtain a
good, complete tensile profile. A curve will result
showing how it reacted to the forces being applied. The point of
failure is of much interest and is typically called its
"Ultimate Strength"
or UTS on the chart.
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Hooke's Law
For most tensile testing of materials, you will notice that in the initial portion of
the test, the relationship between the applied force, or load, and
the elongation the specimen exhibits is linear. In this linear region,
the line obeys the relationship defined as "Hooke's Law"
where the ratio of stress to strain is a constant, or
.
E is the slope of the line in this region where stress (σ) is proportional
to strain (ε) and is called the
"Modulus of Elasticity"
or "Young's Modulus".
Modulus of Elasticity
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The modulus of elasticity is a measure of the stiffness of the material,
but it only applies in the linear region of the curve. If a specimen
is loaded within this linear region, the material will return
to its exact same condition if the load is removed. At the point
that the curve is no longer linear and deviates from the
straightline relationship, Hooke's Law no longer applies and
some permanent deformation occurs in the specimen.
This point is called the "elastic, or
proportional, limit".
From this point on in the tensile test, the material reacts plastically
to any further increase in load or stress. It will not return to its
original, unstressed condition if the load were removed.
Yield Strength
A value called "yield strength"
of a material is defined as the stress applied to the material at which
plastic deformation starts to occur while the material is loaded.
Offset Method
For some materials (e.g., metals and plastics), the departure from
the linear elastic region cannot be easily identified. Therefore,
an offset method to determine the yield strength of the material
tested is allowed. These methods are discussed in ASTM E8 (metals)
and D638 (plastics). An offset is specified as a % of strain (for
metals, usually 0.2% from E8 and sometimes for plastics a value
of 2% is used). The stress (R) that is determined from the
intersection point "r" when the line of the linear
elastic region (with slope equal to Modulus of Elasticity) is
drawn from the offset "m" becomes the
Yield Strength
by the offset method.
Alternate Moduli
The tensile curves of some materials do not have a very welldefined
linear region. In these cases, ASTM Standard E111 provides for
alternative methods for determining the modulus of a material,
as well as Young's Modulus. These alternate moduli are the
secant modulus
and tangent modulus.
Strain
You will also be able to find the amount of stretch or elongation the
specimen undergoes during tensile testing This can be expressed as
an absolute measurement in the change in length or as a relative
measurement called "strain". Strain itself can be expressed
in two different ways, as "engineering strain" and
"true strain". Engineering strain is probably the
easiest and the most common expression of strain used. It is
the ratio of the change in length to the original length, .
Whereas, the true strain
is similar but based on the instantaneous length
of the specimen as the test progresses, , where L_{i}
is the instantaneous length and L_{0} the initial length.
Ultimate Tensile Strength
One of the properties you can determine about a material is
its ultimate tensile strength (UTS).
This is the maximum load the specimen sustains during the test.
The UTS may or may not equate to the strength at break. This all
depends on what type of material you are testing. . .brittle,
ductile,
or a substance that even exhibits both properties.
And sometimes a material may be ductile when tested in a lab,
but, when placed in service and exposed to extreme cold temperatures,
it may transition to brittle behavior.
